Field of the Invention and Description of the Related Art
The present invention relates to a system for navigating a vehicle according to the appearance of a surface region over which it moves, which comprises, on board of the vehicle, a radar altimeter adapted to send signals down towards said surface region and analyse signals reflected therefrom for measuring the height over the part of the surface region just passed, a data base with information about the altitude of the surface region within the area in which the vehicle is present and a device having means adapted to compare the altitude of the surface region last measured with the data base information about surface region altitudes within a given uncertainty area and form a measurement shell extending over this area at the distance measured over the surface region and having a predetermined vertical thickness and having a control unit adapted to control this procedure of forming a measurement shell to be repeated for forming new measurement shells and sections between such shells and the previous measurement shell for determining the position of the vehicle to be within the area of the section, which forms a new measurement shell, and to control a restart of said procedure if it is failed to form said section when attempted to, as well as a method according to the preamble of the appended independent method claim.
Such a system may be used for navigating vehicles of the most different types, for example aircrafts, which may comprise all types of airborne vehicles, such as aeroplanes, unmanned flying objects and so on, in which the navigation is carried out according to the appearance of the terrain overflown by the air-craft, but the vehicle may also be constituted by a boat, a submarine or the like, in which the surface region then is the bottom of the water, such as the sea, in which the boat is moving.
However, the particular case of navigating an aeroplane will be discussed in this disclosure hereinafter so as to illuminate, but accordingly not in any way restrict the invention. It is pointed out that even if the radar altimeter measures the height of the aeroplane above the terrain it is understood that the altitude of the terrain is thereby indirectly determined, since the aeroplane through other sensors has information about the altitude thereof above sea level, so that thanks to the measurement of the radar altimeter said comparison of an altitude of the terrain measured with data base information about terrain altitudes may be carried out.
A system of this type may be used for navigating an aircraft during start and landing as well as cruise flight while utilizing the information given by the appearance of the terrain overflown about the position of the aeroplane.
Said device included in the system is already known under the name terrain navigating filter, and this device performs an algorithm when forming a measurement shell in the way defined above. Such a terrain navigating filter has been developed by the applicant and been in use since 1995. Thus, in such a device or terrain navigating filter an assumption about the noise of the radar altimeter is made, which depends on the fact that the measurement may sometimes take place on objects on the ground, such as trees, and a vertical thickness of the measurement shell is for that sake chosen so that independently of such noises it may for certain be said that the aeroplane is within the measurement shell.
Should the noise be larger, there would be a risk for divergency, i.e., that the aeroplane ends up outside the measurement shell. Should such a divergency occur when the aeroplane flies over flat ground or water the device would not xe2x80x9cdiexe2x80x9d but will continue to deliver erroneous position estimations, since there is then no terrain varying making it possible to get the overlap to disappear. A device overestimating the noise of the radar altimeter is normally used for avoiding this problem, but the costs for this overestimation is that the transient time increases and the performance of the device is deteriorated, i.e. the algorithm gets less exact. By such an increase of the thickness of the measurement shell it is for instance said that xe2x80x9cthe trees are never higher than 50 metersxe2x80x9d, instead of as earlier 25 meters for example.
The object of the present invention is to provide a system and a method of the type defined above, which improve the functionality and the robustness of systems and methods of this type already known.
The object according to the invention is obtained by providing such a system with at least three said devices adapted to carry out said position determination of the vehicle and members adapted to compare the positions determined by the devices and upon a substantial deviation of any position from the other assume that this position is erroneous and form an average of the other positions as the correct position of the vehicle.
By using a plurality of devices or terrain navigating filters in this way the risks of an occurrence of a fault in the position indication gets considerably smaller than when using only one device. Since the aeroplane, in the case of a vehicle in the form of an aeroplane, is only located in one position at a given point of time it may be said that there is only one true position and infinitely many erroneous positions at a given point of time. This means that if a device makes a position determination coinciding with position determinations of other devices these devices have to give correct estimations of the position of the aeroplane, since the probability of the same erroneous position determination of several devices is infinitely low.
According to a preferred embodiment of the invention, the control units of the different devices are adapted to co-operate for reducing the mutual dependency of the position determinations of the devices. This constitutes a very preferred embodiment of the invention, since a high degree of independency of the different devices with respect to each other considerably reduces the risk of two devices indicating the same erroneous position.
A reduction of said dependency may in another preferred embodiment of the invention take place by arranging said means of at least one device to form a measurement shell having a thickness differing from the measurement shell thicknesses of the means of at least another device. By varying adjustments of the noise of the radar altimeter in this way it gets possible to determine whether any device indicates an erroneous position value after running through said procedure as a consequence of a too thin, or too thick, measurement shell, so that the position indication gets inexact.
According to another preferred embodiment of the invention, the different devices are adapted to start said procedure at a time delay with respect to each other, which makes it possible to maintain the robustness also over a terrain having small variations, such as in the aeroplane case travelling over agricultural land or a body of water.
According to another preferred embodiment of the invention, the radar altimeter is adapted to carry out measurements more often than the respective device is adapted to form a new measurement shell, and the respective device is adapted to utilize results from other altitude measurements of the radar altimeter than the other devices. By the fact that the different devices in this way utilize different altitude measurements for the position determination thereof the risk of a repeated error as a consequence of any unusually large obstacle when measuring the terrain is extremely low.
Additional advantages as well as advantageous features of the invention are evident from the following description and the other dependent claims.